1. Field of the Invention
The present invention relates to fully implantable hearing aid system, and more particularly to an apparatus for and method of mounting a microactuator of the fully implantable hearing aid system that permits readily removing the microactuator either permanently or for microactuator replacement.
2. Description of the Prior Art
Patent Cooperation Treaty ("PCT") patent application no. PCT/US96/15087 filed Sep. 19, 1996, entitled "Implantable Hearing Aid" ("the PCT Patent Application") describes an implantable hearing aid which uses a very small implantable microactuator. The PCT Patent Application also discloses a Kynar.RTM. microphone which may be physically separated far enough from the implanted microactuator so that no feedback occurs. A PCT patent application no. PCT/US97/002323 entitled "Improved Biocompatible transducers" filed Feb. 14, 1997, ("the Improved Transducers PCT patent application") discloses improved implantable microactuators and microphones that are useful in the fully implantable hearing aid system disclosed in the PCT Patent Application. The fully implantable hearing aid system disclosed in the PCT Patent Application and in the Improved Transducers PCT Patent Application can operate for a period of five years on a set of batteries, and produce sound levels of 110 dB. The fully implantable hearing aid system described in these PCT Patent Applications is extremely compact, sturdy, rugged, and provides significant progress towards addressing problems with presently available hearing aids.
As described in these PCT Patent Applications, the microactuator is implanted into a fenestration that pierces the promontory of the cochlea. The PCT Patent Applications describes securing the microactuator within this fenestration by screwing the microactuator into the bony wall of the promontory. Fixed in that location the microactuator, either directly or indirectly, excites a basilar membrane in contact with the cochlear fluid, and thereby generates sound. However, over time tissue may grow around the microactuator which anchors it firmly in place, but also making its removal very difficult.
The bone at the promontory of the cochlea is extremely hard, and in some instances is only 0.3 to 0.5 mm thick. The bone's hardness impedes attaching the microactuator with barbs. In some instances, forming screw threads into the bone may also prove difficult because of the promontory's thinness.